Multiband analog front end brings one-chip radio closer to reality
Each of these radio links serves a vital purpose, and leaving one out of the mix would shortchange the operational team. Yet each radio literally carries a cost in size, weight, and spare battery needs. The problem is further complicated as new requirements and links are added to the list.
The solution is obvious, at least on "paper": a universal full-duplex radio module which can be used across all platforms and dynamically reconfigured in the field as needed. The "one-radio" goal would lessen the load, provide flexibility and versatility, be efficient and so provide longer operating life from a single set of batteries, and thus provide significant SWaP (size, weight, power) advantages. That was the underlying premise of programs such as the JTRS (Joint Tactical Radio System) and software-defined radio (SDR) efforts.
But making the "universal" radio concept into a reality has proven harder than envisioned. While Moore’s law has driven the availability of the high-performance, lower-power processors (including FPGAs) which are needed, providing the suitable integrated analog front end (AFE) has been much more difficult. The demands on this functional block–which resides between the antenna and the processor and is the interface between the real-signal world and the digital world–are complex, varied, and stringent.
Until recently, a practical AFE for this type of versatile radio required an array of overlapping parallel channels, each designed to cover a particular segment of the RF spectrum and with bandwidth matched to the intended signal format. This approach, while doable, is costly in terms of final PC board footprint, weight, power, and dollar cost.
A high-performance single-chip AFE solution
Fortunately, a new RF IC is available which addresses the challenging SDR requirements and brings the SDR concept closer to reality. The AD9361 RF Agile Transceiver from Analog Devices is a wideband, programmable front end supporting dual independent transceiver channels, to serve the fast-growing multiple-input, multiple-output (MIMO) segment as well as non-MIMO needs. The system processor can dynamically reconfigure key parameters (such as bandwidth and RF frequency) to match the application needs and thus deliver optimum results. The device also includes features to support frequency agile protocols.
This 10 × 10 mm chip-scale device, Figure 1, has user-tunable bandwidth from 200 kHz to 56 MHz along with other features and performance attributes which are needed to build a signal chain spanning 70 MHz to 6 GHz. Using this 2 × 2 direct-conversion component reduces the entire AFE into a single, relatively simple circuit. It interfaces with the host processor via an LVDS or CMOS port for speed and simplicity. Within the IC are 12-bit A/D and D/A converters, fractional-N synthesizers, digital and analog filters, AGC (automatic gain control), transmit power monitoring, quadrature correction, and other critical functions.
Along with its high level of integration, the RF, analog, and mixed-signal performance is impressive. For example, the receiver noise figure is less than 2.5 dB, while transmitter EVM (error vector magnitude) is better than -40 dB and transmitter noise floor is below -157 dBm/Hz. For both transmit and receive paths, the local oscillator step size is just 2.5 Hz for precise tuning. Despite the many functions within the IC, power consumption is low generally around 1 W.
System design is more than an IC
Since a complex design such as a flexible, wideband SDR involves major circuitry-design effort along with algorithm development and tradeoffs, the AD9361 cones with an available reference design optimized for use with Xilinx FPGAs. The AD-FMCOMMS2-EBZ FMC Board (FPGA Mezzanine Card) from Analog Devices is connected to the Xilinx host board via a single FMC connector, providing power and the bandwidth to support the AD9361 in a 2 × 2 channel configuration, Figure 2. The board is fully customizable in software without any hardware changes and provides additional options for various MIMO configurations.
The reference design includes schematics, layout, BOM, HDL, Linux drivers and application software, all of the key details needed for validating performance and rapid system protoyping. In addition to lower level software and firmware users have the ability to utilize Simulink and MATlab support, enabling code development and tuning of the radio algorithms and performance.
Better than you can do…maybe
Since this small, high-performance, and flexible IC replaces a considerable amount of discrete circuitry, it may seem that the need for such discrete designs is over and done. This is not necessarily so, as a well-designed, carefully debugged, and properly laid-out discrete AFE design for a given segment, format, and bandwidth of the SDR’s total range may be able to outperform the AD9631 IC in that specific segment albeit in a larger footprint.
But the real challenge is the extremely broadband nature of the AFE n the SDR, which would need many such spectrum-specific front ends, each of which is a large challenge to design and evaluate, and the final product will fall far short in the SWaP ranking. Therefore, the tradeoff tilts heavy in favor of the AD9361 IC, with RF performance which is more than sufficient for most situations, and with far fewer shortcomings.
Analog Devices’ AD9361 RF Agile Transceiver is not just another "coming soon" IC supported by high hopes and an optimistic data sheet. The IC is real, the FMC board and tools are real, and it is already designed into two available SDR products, the Universal Software Radio Peripheral (USRP) from Ettus Research (www.ettus.com) and the Maveriq™ Multichannel Reconfigurable RF Transceiver from Epiq Solutions (www.epiqsolutions.com).
Whether system engineers prefer to do their SDR design and development using the Analog Devices FMC, or with a commercially available SDR as the platform, the overall product package and performance based on the AD9361 will give them a major head start.
About the author
Duncan Bosworth is Analog Devices’ Segment Marketing Engineer for the Aerospace and Defense Segment. He is based in the company’s Wilmington, MA, USA facility.
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